The present invention relates in general to frequency modulation systems, and in particular to a digital frequency modulation system in which the low frequency component of a modulating signal is subjected to conventional frequency modulation and the high frequency component of the modulating signal is subjected to narrow-band frequency modulation.
Increases in the speed of integrated circuits have contributed to the rapid development of videotape recorders. Integrated circuits now operate at speeds that allow for video signals to be digitized. A composite video signal, which, contains information on luminance, is digitized and then stored on a magnetic tape by the videotape recorder. For instance, the red, green and blue components of the composite signal are stored on separate tracks of the magnetic tape. Since the video signal is amplitude modulated, however, each component of the composite signal must first be frequency modulated before being stored on the magnetic tape.
A conventional modulator system is disclosed in an article entitled "Digitalisierung der Video-Singalverarbeitung Beispiel: Video-Kassettenrecorder" by Peter Draheim in the German publication Elektronik-Sonderheft, No. 57, 1984 on pp. 97-100. A first adder adds a modulating signal to a carrier signal for output to an integrator. The integrator contains a second adder, a memory element and a feedback path, which couples the output of the memory element to a first input of the second adder. The output of the first adder is coupled to a second input of the second adder. The output of the memory element, which also provides the output of the integrator, is applied to a sine lookup table, whose output provides a frequency modulated signal.
During conventional frequency modulation, the frequency of the modulating signal must fall within the band fc.+-..DELTA.f, where fc is the carrier frequency and .DELTA.f is the frequency deviation. If the modulating signal contains high frequency components as compared to the sampling frequency, high sidebands will be produced outside the band. Frequencies of these sidebands may fall back into the band and occur as "alias signals." Once in the band, the noise caused by these alias signals cannot be eliminated by subsequent filtering.
It is well known to modulate a carrier signal by splitting the modulating signal into its low-frequency and high-frequency components. For instance, in U.S. Pat. No. 3,868,599 entitled "SINGLE SIDEBAND FREQUENCY MODULATION SYSTEM" and issued to Takashi Hiraski and Gojiro Suga on Feb. 25, 1975, a modulating signal is split into its low-frequency and high-frequency components by low-pass and high-pass filters, respectively. The output of the high-pass filter is phase-shifted, integrated and applied to an exponential function generator. The output of the high-pass filter is also combined with the low-frequency component and frequency modulated by a conventional frequency modulator. The respective outputs of the exponential function generator and frequency modulator are combined in an amplitude modulator. Thus, by removing the low-frequency component from the exponential function generator, the range of outputs of the exponential function generator remains relatively stable and, therefore, the exponential generator does not become saturated. However, since the high-frequency component of the modulating signal is used to modulate the carrier signal by conventional means, the alias signals still occur in the band.
One example of apparatus for reducing the noise caused by alias signals is disclosed in U.S. Pat. No. 4,802,016 entitled "VIDEO NOISE REDUCTION SYSTEM WITH SIGNAL EMPHASIS PRECEDING FM MODULATION UPPER-BAND LUMINANCE" and issued to Shinji Kaneko on Jan. 31, 1989. Prior to FM modulation, a luminance signal is split into its high-frequency and low-frequency components. The low-level components of the high-frequency luminance signal are emphasized by an emphasis circuit and are then added to the low frequency luminance signal by an adder. The output of the adder is then frequency modulated by conventional means.
Therefore, it is an object of the present invention to provide a frequency modulation system that provides a modulated signal which is free of alias signals.
It is a further object of the present invention to provide a digital frequency modulator that splits a modulating signal into its high-frequency and low-frequency components, modulates the low-frequency component by conventional means, narrow-band modulates the high-frequency component, and then adds the modulated components together.